Such a transponder for inductive transmission of energy signals and/or data signals is described in U.S. Pat. No. 5,491,483, for example. Provision is made therein for a base station that generates a magnetic alternating field via which a transponder oscillating circuit of the transponder is excited. The excited transponder oscillating circuit supplies a device for charging an energy store of the transponder, e.g. a capacitor. For the subsequent transmission of energy or data from the transponder to the base station, the transponder oscillating circuit is excited to resonate on its specific internal frequency by means of a starter device. The energy which is required for covering in particular the attenuation losses of the transponder oscillating circuit is extracted from the energy store of the transponder.
In existing systems, the energy extraction is also controlled from the energy store in such a way that the signal or voltage amplitude of the transponder oscillating circuit when sending is significantly smaller at the end of the protocol than at the beginning: In the case of single-stage transponder systems, this is a minor problem due to the close coupling, since the signal amplitude is generally very high. In the case of two-stage systems, however, this frequently results in undesirably small signal/noise ratios towards the end of the protocol or transmission due to the low signal amplitudes. The smaller the signal/noise ratio during the send operation from the transponder, the greater the probability of error when detecting the sent signals at the receiver. This is described below with reference to FIGS. 1 and 2.
FIG. 1 shows a schematic block diagram of a generally disclosed transponder (T) 1. The known transponder (T) 1 for inductive sending and receiving of energy signals and/or data signals has a transponder oscillating circuit (TSK) 2 which is coupled inductively to a base station (BS) 20 on one side and via a starter device (AV) 3 to a storage capacitor (SK) 13 on the other side. When energy and/or data are/is received, the storage capacitor (SK) 13 is charged with an initial charging voltage UCl. When sending, the storage capacitor (SK) 13 is discharged. During the discharge operation, the discharge voltage UCe at the storage capacitor (SK) 13 drops. At the start of sending t0, the amount of the discharge voltage UCe corresponds to the amount of the initial charging voltage UCl.
When sending, the transponder oscillating circuit (TSK) 2 is supplied with energy from the storage capacitor (SK) 13 by means of the discharge operation if the send voltage US-TSK of the transponder oscillating circuit (TSK) 2 is less than a send voltage desired value US-SOLL of the starter device (AV) 3. The send voltage desired value of the starter device US-SOLL is defined as the difference between the present discharge voltage UCe and a reference voltage Uref which is permanently predefined by the starter device (AV) 3.
FIG. 2a shows a U/t diagram for illustrating the time-based profile of the send voltage US-TSK and the discharge voltage UCe during a send operation for a transponder (T) 1 as per FIG. 1. The send duration ts is defined as the period between the start of sending t0 and the end of sending tE.
At the start of sending t0, the amount of the discharge voltage UCe is identical to the amount of the initial charging voltage UCl. The discharge voltage UCe of the storage capacitor (SK) 13 decreases exponentially in its time-based profile. The curve of the send voltage desired value US-SOLL is given by the difference between the curve of the discharge voltage UCe and the amount of the reference voltage Uref. If the send voltage actual value US-IST falls below the send voltage desired value US-SOLL, the starter device (AV) 3 excites the transponder oscillating circuit (TSK) 2 by means of the discharge current ICc. The excitation of the send voltage US-TSK of the transponder oscillating circuit (TSK) 2 by means of the discharge current ICe is generally referred to as “plucking”.
As a result of the exponential subsidence of the discharge voltage UCc, the send voltage actual value US-IST will always fall below the send voltage desired value US-SOLL as from the time point tZ, wherein the time point tZ designates the time point from which the discharge voltage UCe is less than or equal to the reference voltage Uref. The consequence of this is a constant “plucking” between the time points tZ and the end of sending tE. If the discharge voltage UCe reaches the reference voltage Uref before the end of sending tE, the amplitude of the envelopes H of the send voltage US-TSK is no longer sufficient for sending with regard to the signal/noise ratio. As shown in FIG. 2a, the signal/noise ratio for the send voltage US-TSK between the time points tZ and the end of sending tE will be too small.
FIG. 2b shows an I/t diagram for illustrating the discharge current ICe during a send operation for a transponder (T) 1 as per FIG. 1. A current pulse of the discharge current ICe is generated At the time points t0, t1, t2, etc., thereby exciting the transponder oscillating circuit (TSK) 2 as per FIG. 1 and FIG. 2a. The distance separating the “plucking” or current pulses becomes steadily smaller between the time points t0 and tz, since the discharge voltage UCe and therefore the send voltage desired value US-SOLL of the starter device (AV) 3 decrease exponentially. The envelope H of the send voltage US-TSK therefore subsides exponentially (see FIG. 2a, FIG. 2b), i.e. most of the signal energy or signal amplitude is available at the start of sending t0. The signal amplitude or envelope H of the send voltage US-TSK continues to subside during the send duration ts, and consequently the signal/noise ratio becomes continuously smaller.
If the time point tZ comes before the end of sending tE, the signal which must be sent has too little signal energy in the time window between tZ and tE. Too little signal energy with regard to the existing physical send channel results in a signal/noise ratio which is too small to allow correct detection of the sent signal at the receiver.
If the time point tZ comes after the end of the signal duration tE, however, all the energy which would have been available for sending is not fully utilized. Over the whole send duration ts, therefore, a reduced signal energy is provided for the send voltage US-TSK, even though the storage capacitor (SK) could make more energy available for sending and therefore for increasing the signal/noise ratio. Consequently, optimal utilization of the energy provided if applicable by the storage capacitor is not established.